Recent years have seen higher resolution and more gray-levels in display devices, of which flat panel displays are representative, to allow images to be presented to a user with higher picture quality and a greater sense of realism. In addition, the video signals supplied to display devices have become digitized, typically with data being allotted at six bits per pixel to eight bits per pixel for each of the R (red), G (green), and B (blue) color components.
When the number of bits of the video signal and the number of bits that can be displayed on the display device are the same, the input signal is basically used as is to display on the display device. However, the number of bits of the video signal received as input frequently differs from the number of bits that can be displayed on the display device. When the number of bits of the video signal is greater than the number of bits that can be displayed on the display device, the number of bits of the video signal are reduced by using, for example, a method in which the lower-order bits are discarded, a dither method, or an FRC (Frame Rate Control) method.
On the other hand, when the number of bits of the video signal is less than the number of bits that can be displayed on the display device, the number of bits of the video signal is increased by adding lower-order bits (a gray-level extension process). This gray-level extension process is also used when data processing of a video signal according to characteristics of a display device is executed in the display device itself. The gray-level extension process is also executed to increase the arithmetic precision even when the number of bits of the video signal is the same as the number of bits that can be displayed on the display device. In this case, the above-described dither method or FRC is used to convert the video signal to the number of bits that can be displayed on the display device after the gray-level extension process has been executed.
The gray-level extension process has also been used for purposes other than converting the number of bits of the above-described digital signal or raising arithmetic accuracy. For example, when the number of bits of the digital signal is low, false contour (although gray-level should change smoothly within a plane, the portion of gray-level change is not perceived to change smoothly and a contour is discernible) become visible in regions in which gray-level changes smoothly such as in gradation. Gray-level extension processing is also used as a technique for preventing such false contours.
Generally, the gray-level extension process can be divided between two types: (1) a process for implementing the same process on all video signals; and (2) a process for extracting the video signal of a specific images and implementing processing only for necessary pixels.
As a method of performing the same processing on all video signals, a method can first be considered in which dither noise or random noise is added. Although this method can suppress false contours to a degree, it entails the problem of a noticeable added noise component.
In a second method, the values of higher-order bits are added as lower-order bits. For example, to convert a six-bit input signal “101101” to eight bits, the values of the highest two bits are added to the two lower-order bits to convert to “10110110.”
As a third method, “0” or “1” is simply added to the lower-order bits.
The second and third methods are simple, but because the gray-level differences at points of gray-level change are not reduced, these methods cannot suppress false contours.
In contrast, as a method for (2) extracting video signals of specific images and performing processing only for necessary pixels, Japanese Patent Laid-Open No. S63-15576 (hereinbelow referred to as Patent Document 1) discloses a method of implementing a low-pass filter (LPF) process on false-contour regions. In this technique of the related art, regions in which false contours occur due to the implementation of gamma correction of a digital image signal (image processing) are adaptively determined in order to suppress the generated false contours, and in these regions, the integral value (the same meaning as the LPF process) of the video signals of neighboring pixels is supplied as output. This LPF process reduces the gray-level differences generated by false contours. However, in this method, when the spacing of generation of false contours (i.e., the spacing of contour lines) is greater than the filter size (the integral range of neighboring pixels), differences can be discerned in regions of gradation between sites at which the filter process is carried out and other sites in which the filter process is not carried out despite the suppression of false contours, and as a result, the method does not realize a marked improvement in image quality.
In response, a method is disclosed in Japanese Patent Laid-Open No. H04-165874 (hereinbelow referred to as Patent Document 2) as a second method in which, when regions in which change in gray-level is smooth (gradation region) are determined for the purpose of suppressing false contours that have been generated by the implementation of gamma correction (image processing), the gray-level values of pixels between contour lines of false contours in these regions are found by linear interpolation of the gray-level values of pixels on contour lines. This method enables uniform gray-level changes in gradation regions and the problems of the first method therefore do not arise.
As can be understood from the foregoing explanation, as the method of a gray-level extension process, a method in which specific information of pixels is detected and linear interpolation then carried out in accordance with the results of detection is preferable from the standpoint of suppressing false contours. Methods that use this linear interpolation are also disclosed in, for example, Japanese Patent Laid-Open No. 2003-304400 (hereinbelow referred to as Patent Document 3), Japanese Patent Laid-Open No. 2003-333348 (hereinbelow referred to as Patent Document 4), and Japanese Patent Laid-Open No. 2004-54210 (hereinbelow referred to as Patent Document 5).
In Patent Documents 3-5, the linear interpolation method is the same as in Patent Document 1 and Patent Document 2, but in contrast to Patent Document 1 and Patent Document 2, the bit depth of the digital image signal is shallow, and as a result, gray-level extension processing is carried out to solve the problem of false contour generation or to elicit the maximum gray-level performance of the display device.
The image processor disclosed in Patent Document 3 is of a configuration that includes a false contour detection unit and a pixel value converter.
This false contour detection unit takes as the detection conditions of a false contour a case in which the luminance level increases by “1” after the same luminance level has continued in the horizontal direction for two or more pixels (condition 1) and a case in which the same luminance level has continued two or more pixels in the horizontal direction after the luminance level has dropped by “1”. The false contours that have been detected are then subjected to linear interpolation in the pixel value converter.
The color-signal extending apparatus disclosed in Patent Document 4 is of a configuration that includes a data distribution detection section and a data depth extending section. The data distribution detection section extracts (detects) regions in which color changes smoothly from the distribution state of gray-level data. More specifically, the data distribution detection section detects regions in which, in a pixel group K in which the same gray-level continues, the number of pixels is at least a minimum threshold value P and no greater than a maximum threshold value Q, and moreover, in which the gray-level difference with pixels of adjacent pixel groups is no greater than a determination threshold value S. The data distribution detection section then carries out the linear interpolation for regions of smooth change and determines the gray-level values that are added to these regions. The data depth extending section generates extended image data in which the data depth of the color signal is extended while adding the gray-level values to the extended portion.
The image processor disclosed in Patent Document 5 is of a configuration that includes a detecting means and a signal expanding means. The detecting means determines whether false contours exist by determining whether the difference between the first position in which the same pixel data continues and the first position at which the next pixel data continues is equal to the width over which the same pixel data continues, and further, by determining whether the gray-level value of a region in which the same pixel data continues is one greater or one smaller than the gray-level value of the next region in which the pixel data continues. Gray-level expansion is next carried out smoothly and linearly (by implementing linear interpolation) in the signal expanding means such that an image is obtained that continues smoothly in a region in which a false contour occurs.
However, in the inventions disclosed in the above-described Patent Documents 3-5, the process of detecting false contours is carried out only for a particular direction (for example, the horizontal direction) on the image plane, but false contours must be detected for two directions to obtain an adequate effect by means of the gray-level extension process. The problem therefore arises that a frame memory becomes necessary for holding the pixel data of a one-screen portion, thereby entailing higher costs. In addition, although a method can be considered for using a line memory that costs less than a frame memory, the use of a line memory allows detection of false contours in only one direction (for example, the horizontal direction) as in the inventions disclosed in Patent Documents 3-5 described hereinabove. In other words, the gray-level extension process is carried out for each line of an image.
As described hereinabove, carrying out the process of detecting false contours for each of two directions in an image processor of the related art necessitates the use of a frame memory and therefore raises the problem of increased costs. On the other hand, the use of a line memory to limit increase in costs means that the process of detecting false contours and the process of extending gray-levels are carried out for each line of an image, whereby the correlation between each line of an image is not considered. The problems therefore arise that linear noise of images becomes visible and a desirable gray-level extension process cannot be executed.